As telecommunication technologies advance from wired to wireless communication driven by efficiency and convenience for the general public in the past decade, wireless communication devices and their implementation have become ubiquitous. Antennas have been a key building block in the construction of every wireless communication system. In many instances, the antenna is not considered critical in the initial system design. However, the antenna is the single device that allows RF energy to transmit between wired transmission lines and free space. Consequently, antennas and propagation are the key factors influencing the robustness and quality of the wireless communication channel.
Typically, conventional helical antennas or linear monopole antennas are used as antennas for portable terminals. The helical antennas or linear monopole antennas have a merit of omni-directional radiation characteristic, since they are of external type projecting outside the device, therefore, they are likely to be damaged by an external force.
One planar antenna called planar inverted F antenna (PIFA) having a low profile structure is employed as an internal antenna configured inside a mobile communication terminal. The conventional PIFA includes a radiating element, a coaxial wire and a ground plane. The radiating element is fed through the coaxial wire, and is connected to the ground plane so that an impedance match can be achieved. The conventional PIFA must be designed by taking into account the length L of the radiating element and the height of the antenna according to the width of the radiating element. The PIFA functions as a square-shaped micro-strip antenna with the length of the radiating unit reduced to half, achieving a low profile structure. Further the PIFA is an internal antenna installed in the mobile communication terminal, thereby being aesthetically designed and protected from external impact.
Further, the ground plane of the antenna plays a significant role in its operation. Excitation of currents in the printed IFA causes excitation of currents in the ground plane. The resulting electromagnetic field is formed by the interaction of the IFA and an image of itself below the ground plane. Its behavior as a perfect energy reflector is consistent only when the ground plane is infinite or very much larger in its dimensions than the monopole itself. In practice the metallic layers are of comparable dimensions to the monopole and act as the other part of the dipole.
Since the miniaturization method used in the conventional antenna is based on a two-dimensional structure, there is a limit to the miniaturization. The space for the antenna in the portable device is reduced day by day, there is a keen need of improvement for the miniaturization. There is still a need of improvement in view of a space use or a feeding efficiency.
However, wireless communication is characterized by limited available frequency spectrum, low transmission powers and limited device processing capability. Furthermore, narrow bandwidth characteristic of conventional PIFA is one of the limitations for its commercial application for wireless mobile at present.